The present invention relates to the measurement of the recombination lifetime of excess or injected carriers in semiconductor devices such as diodes, transistors, thyristors and similar devices, and provides a system for making such measurements and providing a direct visual digital display of the lifetime.
The recombination time, or lifetime, of excess carriers in the high resistivity regions of semiconductor devices is a parameter of great importance in the design and manufacture of such devices. Thus, in the case of high power thyristors, for example, the value of these devices in primarily dependent on a high level bulk lifetime. In such devices, excess carriers are injected into the high resistivity or base region during the ON state of the device and should have a high lifetime in order to sufficiently flood this region with carriers to keep the resistivity low, so that the voltage drop across it is as low as possible. This is necessary to keep the power dissipation as low as possible, since the ratings, and usefulness, of these devices are limited by the power dissipated in the forward conducting or ON state. Power thyristors may carry currents of hundreds, or even thousands, of amperes so that a forward drop as low as one volt may represent a power dissipation of one or more kilowatts. This results in such devices being large and heavy with massive heat sinks to prevent excessive temperature rise due to the power dissipation. It is essential, therefore, to keep the power dissipation low which requires that the resistivity and forward voltage drop be as low as possible.
Thyristors are usually turned OFF by self-commutation; that is, when the current falls below a minimum level, the internal potentials fall low enough so that the high resistivity region and its associated junction return to the equilibrium condition, and the device reverts to the high impedance or blocking state. In the absence of a forced reverse current, this can only happen when the excess carriers injected during the ON condition are reduced to near normal levels by recombination of holes and electrons in the bulk. This process takes an appreciable amount of time because of the high density of injected carriers during the ON state which may be reduced by approximately 12 or 13 orders of magnitude. The rate of which the carrier density is reduced is determined by the bulk lifetime and generally some tens of lifetimes are required to reach the OFF state.
It will be apparent, therefore, that a short carrier lifetime is required for low turn-off time, which is usually highly desirable or even essential to improve the frequency response, while a high carrier lifetime is needed to minimize the forward voltage drop and power dissipation during the ON state. These requirements of high carrier lifetime to minimize forward voltage drop and short carrier lifetime to decrease the turn-off time are obviously directly conflicting. This conflict can be resolved, however, by careful design of the device involving well balanced trade-offs to achieve satisfactory compromises for particular types of devices or for particular applications. Such design practice requires tight and effective control in the manufacture of the devices to maintain the desired values and achieve the intended design objectives at a reasonable cost.
Such control requires the use of some means for measuring the excess carrier lifetime in a reasonably rapid and accurate manner, not only for design and development purposes, but also as a continuing routine production check, or quality control test, to insure that the necessary standards are being maintained. The carrier recombination lifetime can be determined by measuring the slope of the open-circuit voltage decay curve across a semiconductor device following the termination of a current pulse, as discussed, for example, in Mazur U.S. Pat. No. 3,697,873. This has been done heretofore by displaying the voltage decay curve on an oscilloscope screen and directly measuring, or scaling-off, the slope of a predetermined portion of the curve from which the lifetime can be calculated. It is obvious that this is a cumbersome and unsatisfactory procedure for use as a routine production test, as it is difficult and time-consuming as well as being subject to operator error especially when carried out by semi-skilled production personnel.